package main

import (
	"bytes"
	"errors"
	"fmt"
	"log"
	"math"
	"os"
	"path/filepath"
	"regexp"
	"sort"
	"strconv"
	"strings"
	"sync"
	"time"

	"github.com/emersion/go-ical"
	"github.com/teambition/rrule-go"
)

// calendarsDir is the absolute path of the calendars folder (-calendars).
// Empty disables the diary lines, the maintenance widget and the sync. Every
// calendar is one <Name>.ics file in this folder; the folder's .page-settings
// assigns calendars to roles and holds the CalDAV sync credentials.
var calendarsDir string

// === Settings ===

type calendarSettings struct {
	// Roles: calendar names (file basenames without .ics).
	Diary       []string
	Weight      []string
	Maintenance []string

	CaldavURL    string
	CaldavUser   string
	CaldavPass   string
	SyncInterval time.Duration // 0 = startup and manual only
}

// readCalendarSettings reads the calendar keys from dir's .page-settings.
// Read on every use, so edits apply without a restart.
func readCalendarSettings(dir string) calendarSettings {
	kv := readSettingsFile(dir)
	s := calendarSettings{
		Diary:        splitCalendarList(kv["diary"]),
		Weight:       splitCalendarList(kv["weight"]),
		Maintenance:  splitCalendarList(kv["maintenance"]),
		CaldavURL:    kv["caldav-url"],
		CaldavUser:   kv["caldav-user"],
		CaldavPass:   kv["caldav-pass"],
		SyncInterval: 15 * time.Minute,
	}
	if v, ok := kv["sync-interval"]; ok {
		if d, err := time.ParseDuration(v); err == nil && d >= 0 {
			s.SyncInterval = d
		}
	}
	return s
}

func splitCalendarList(v string) []string {
	var out []string
	for _, name := range strings.Split(v, ",") {
		if name = strings.TrimSpace(name); name != "" {
			out = append(out, name)
		}
	}
	return out
}

// calendarPath returns the file of calendar name in dir, or "" when the name
// would escape the folder.
func calendarPath(dir, name string) string {
	if name == "" || strings.ContainsAny(name, `/\`) || strings.HasPrefix(name, ".") {
		return ""
	}
	return filepath.Join(dir, name+".ics")
}

// === Model ===

// calEvent is one expanded occurrence.
type calEvent struct {
	Calendar string
	UID      string
	Summary  string
	Start    time.Time // local time; all-day events start at local midnight
	End      time.Time // exclusive
	AllDay   bool
	Stamp    time.Time  // LAST-MODIFIED, else DTSTAMP
	Series   *calSeries // nil for single events
}

// calSeries describes a recurring event as a whole.
type calSeries struct {
	Calendar string
	UID      string
	Summary  string
	Rule     string // human-readable, e.g. "every 2 weeks (Sa)"
}

// calItem is a parsed VEVENT: a single event, a recurring master, or a
// RECURRENCE-ID override attached to its master.
type calItem struct {
	uid       string
	summary   string
	start     time.Time
	end       time.Time
	allDay    bool
	days      int // all-day length in days; keeps occurrences DST-safe
	stamp     time.Time
	cancelled bool

	set       *rrule.Set         // nil = single event
	series    *calSeries         // set together with set
	overrides map[int64]*calItem // RECURRENCE-ID (unix) → replacement
}

// calFile is one parsed calendar file.
type calFile struct {
	modTime time.Time
	size    int64
	items   []*calItem
	events  int // VEVENT components in the file
}

// === Store ===

// calStore caches parsed calendar files by path. An entry is reused while the
// file's modtime and size are unchanged, so a request costs one stat per
// calendar. Parsed files are never mutated, so callers may keep using a
// snapshot after the lock is released.
var calStore = struct {
	sync.Mutex
	files map[string]*calFile
}{files: map[string]*calFile{}}

// loadCalendar returns the parsed calendar at path; nil when it does not exist
// or cannot be parsed (logged).
func loadCalendar(path string) *calFile {
	info, err := os.Stat(path)
	if err != nil {
		return nil
	}
	calStore.Lock()
	defer calStore.Unlock()
	if f := calStore.files[path]; f != nil && f.modTime.Equal(info.ModTime()) && f.size == info.Size() {
		return f
	}
	data, err := os.ReadFile(path)
	if err != nil {
		return nil
	}
	f, err := parseCalendar(filepath.Base(path), data)
	if err != nil {
		log.Printf("calendar %s: %v", path, err)
		delete(calStore.files, path)
		return nil
	}
	f.modTime = info.ModTime()
	f.size = info.Size()
	calStore.files[path] = f
	return f
}

// calendarOccurrences expands every event of the named calendars in dir that
// overlaps [from, to). The result is sorted by start, then summary.
func calendarOccurrences(dir string, names []string, from, to time.Time) []calEvent {
	var out []calEvent
	for _, name := range names {
		p := calendarPath(dir, name)
		if p == "" {
			continue
		}
		f := loadCalendar(p)
		if f == nil {
			continue
		}
		for _, it := range f.items {
			out = it.expand(name, from, to, out)
		}
	}
	sort.SliceStable(out, func(i, j int) bool {
		if !out[i].Start.Equal(out[j].Start) {
			return out[i].Start.Before(out[j].Start)
		}
		return out[i].Summary < out[j].Summary
	})
	return out
}

func (it *calItem) occurrence(calendar string, start, end time.Time) calEvent {
	return calEvent{
		Calendar: calendar,
		UID:      it.uid,
		Summary:  it.summary,
		Start:    start.In(time.Local),
		End:      end.In(time.Local),
		AllDay:   it.allDay,
		Stamp:    it.stamp,
	}
}

// overlaps reports whether [start, end) intersects [from, to). Zero-length
// events count when they start inside the window.
func overlaps(start, end, from, to time.Time) bool {
	if !start.Before(to) {
		return false
	}
	return end.After(from) || !start.Before(from)
}

// expand appends the item's occurrences in [from, to) to out.
func (it *calItem) expand(calendar string, from, to time.Time, out []calEvent) []calEvent {
	if it.cancelled {
		return out
	}
	if it.set == nil {
		if overlaps(it.start, it.end, from, to) {
			out = append(out, it.occurrence(calendar, it.start, it.end))
		}
		return out
	}
	// Start the search early enough to catch occurrences that began before
	// the window but still run into it.
	after := from.Add(-it.end.Sub(it.start)).Add(-48 * time.Hour)
	for _, s := range it.set.Between(after, to, true) {
		if _, overridden := it.overrides[s.Unix()]; overridden {
			continue
		}
		e := it.occurrenceEnd(s)
		if overlaps(s, e, from, to) {
			ev := it.occurrence(calendar, s, e)
			ev.Series = it.series
			out = append(out, ev)
		}
	}
	// Overrides are placed on their own dates: a moved occurrence can enter
	// or leave the window independently of its original slot.
	for _, o := range it.overrides {
		if o.cancelled || !overlaps(o.start, o.end, from, to) {
			continue
		}
		ev := o.occurrence(calendar, o.start, o.end)
		ev.Series = it.series
		out = append(out, ev)
	}
	return out
}

// occurrenceEnd returns the end of the occurrence starting at s.
func (it *calItem) occurrenceEnd(s time.Time) time.Time {
	if it.allDay {
		return s.AddDate(0, 0, it.days)
	}
	return s.Add(it.end.Sub(it.start))
}

// === Parsing ===

// calParser holds per-file parse state: the file name for log messages, the
// TZIDs already warned about, and resolved locations.
type calParser struct {
	file   string
	zones  map[string]*time.Location
	warned map[string]bool
}

func parseCalendar(file string, data []byte) (*calFile, error) {
	data = bytes.TrimPrefix(data, []byte{0xEF, 0xBB, 0xBF}) // UTF-8 BOM
	cal, err := ical.NewDecoder(bytes.NewReader(data)).Decode()
	if err != nil {
		return nil, err
	}
	p := &calParser{file: file, zones: map[string]*time.Location{}, warned: map[string]bool{}}
	f := &calFile{}
	masters := map[string]*calItem{}
	type override struct {
		item    *calItem
		recurID time.Time
	}
	var overrides []override
	for _, comp := range cal.Children {
		if comp.Name != ical.CompEvent {
			continue
		}
		f.events++
		it, recurID, err := p.item(comp)
		if err != nil {
			continue
		}
		if !recurID.IsZero() {
			overrides = append(overrides, override{it, recurID})
			continue
		}
		f.items = append(f.items, it)
		if it.uid != "" {
			masters[it.uid] = it
		}
	}
	// Overrides can come before their master in the file, so attach them in
	// a second pass. One without a recurring master is shown as a single event.
	for _, o := range overrides {
		m := masters[o.item.uid]
		if m == nil || m.set == nil {
			f.items = append(f.items, o.item)
			continue
		}
		if o.item.summary == "" {
			o.item.summary = m.summary
		}
		m.overrides[o.recurID.Unix()] = o.item
	}
	return f, nil
}

// item parses one VEVENT. recurID is non-zero for RECURRENCE-ID overrides.
func (p *calParser) item(comp *ical.Component) (it *calItem, recurID time.Time, err error) {
	startProp := comp.Props.Get(ical.PropDateTimeStart)
	if startProp == nil {
		return nil, time.Time{}, errors.New("no DTSTART")
	}
	starts, allDay, err := p.times(startProp)
	if err != nil || len(starts) == 0 {
		return nil, time.Time{}, fmt.Errorf("DTSTART: %v", err)
	}
	it = &calItem{start: starts[0], allDay: allDay}
	it.uid = calText(comp.Props.Get(ical.PropUID))
	it.summary = strings.TrimSpace(calText(comp.Props.Get(ical.PropSummary)))

	it.end = it.start
	if prop := comp.Props.Get(ical.PropDateTimeEnd); prop != nil {
		if ends, _, err := p.times(prop); err == nil && len(ends) > 0 {
			it.end = ends[0]
		}
	} else if prop := comp.Props.Get(ical.PropDuration); prop != nil {
		if d, err := prop.Duration(); err == nil {
			if allDay {
				it.end = it.start.AddDate(0, 0, int(math.Round(d.Hours()/24)))
			} else {
				it.end = it.start.Add(d)
			}
		}
	} else if allDay {
		it.end = it.start.AddDate(0, 0, 1)
	}
	if it.end.Before(it.start) {
		it.end = it.start
	}
	if allDay {
		it.days = calendarDaysBetween(it.start, it.end)
		if it.days < 1 {
			it.days = 1
		}
		it.end = it.start.AddDate(0, 0, it.days)
	}

	if strings.EqualFold(calText(comp.Props.Get(ical.PropStatus)), "CANCELLED") {
		it.cancelled = true
	}
	for _, name := range []string{ical.PropLastModified, ical.PropDateTimeStamp} {
		if prop := comp.Props.Get(name); prop != nil {
			if ts, _, err := p.times(prop); err == nil && len(ts) > 0 {
				it.stamp = ts[0]
				break
			}
		}
	}

	if prop := comp.Props.Get(ical.PropRecurrenceID); prop != nil {
		ids, _, err := p.times(prop)
		if err != nil || len(ids) == 0 {
			return nil, time.Time{}, fmt.Errorf("RECURRENCE-ID: %v", err)
		}
		return it, ids[0], nil
	}

	rule := comp.Props.Get(ical.PropRecurrenceRule)
	rdates := comp.Props.Values(ical.PropRecurrenceDates)
	if rule == nil && len(rdates) == 0 {
		return it, time.Time{}, nil
	}
	set := &rrule.Set{}
	ruleText := ""
	if rule != nil {
		opt, err := rrule.StrToROptionInLocation(rule.Value, it.start.Location())
		if err != nil {
			p.warn("rrule:"+rule.Value, "cannot parse RRULE %q of %q: %v", rule.Value, it.summary, err)
			return it, time.Time{}, nil
		}
		opt.Dtstart = it.start
		r, err := rrule.NewRRule(*opt)
		if err != nil {
			p.warn("rrule:"+rule.Value, "cannot build RRULE %q of %q: %v", rule.Value, it.summary, err)
			return it, time.Time{}, nil
		}
		set.RRule(r)
		ruleText = describeRRule(opt)
	} else {
		// Without an RRULE the set only holds RDATEs; DTSTART is always the
		// first occurrence.
		set.RDate(it.start)
		ruleText = "on selected dates"
	}
	for i := range rdates {
		if ts, _, err := p.times(&rdates[i]); err == nil {
			for _, t := range ts {
				set.RDate(t)
			}
		}
	}
	for _, prop := range comp.Props.Values(ical.PropExceptionDates) {
		if ts, _, err := p.times(&prop); err == nil {
			for _, t := range ts {
				set.ExDate(t)
			}
		}
	}
	it.set = set
	it.overrides = map[int64]*calItem{}
	it.series = &calSeries{
		Calendar: strings.TrimSuffix(p.file, ".ics"),
		UID:      it.uid,
		Summary:  it.summary,
		Rule:     ruleText,
	}
	return it, time.Time{}, nil
}

// calText unescapes a TEXT value. Unlike go-ical's Text it keeps unescaped
// commas: clients write titles like "95,6kg" without the required backslash,
// and go-ical would cut them at the comma. Unknown escapes are kept as is.
func calText(prop *ical.Prop) string {
	if prop == nil {
		return ""
	}
	v := prop.Value
	if !strings.Contains(v, `\`) {
		return v
	}
	var sb strings.Builder
	for i := 0; i < len(v); i++ {
		c := v[i]
		if c != '\\' || i+1 == len(v) {
			sb.WriteByte(c)
			continue
		}
		i++
		switch n := v[i]; n {
		case 'n', 'N':
			sb.WriteByte('\n')
		case '\\', ';', ',':
			sb.WriteByte(n)
		default:
			sb.WriteByte('\\')
			sb.WriteByte(n)
		}
	}
	return sb.String()
}

// times parses a DATE or DATE-TIME property. Comma-separated lists (EXDATE,
// RDATE) yield several values. Dates and floating times use the local zone; an
// unknown TZID falls back to it with a warning.
func (p *calParser) times(prop *ical.Prop) ([]time.Time, bool, error) {
	loc := time.Local
	if tzid := prop.Params.Get(ical.PropTimezoneID); tzid != "" {
		loc = p.zone(tzid)
	}
	var out []time.Time
	allDay := false
	for _, v := range strings.Split(prop.Value, ",") {
		v = strings.TrimSpace(v)
		var t time.Time
		var err error
		switch {
		case len(v) == 8:
			t, err = time.ParseInLocation("20060102", v, time.Local)
			allDay = true
		case len(v) == 16 && strings.HasSuffix(v, "Z"):
			t, err = time.Parse("20060102T150405Z", v)
		case len(v) == 15:
			t, err = time.ParseInLocation("20060102T150405", v, loc)
		default:
			err = fmt.Errorf("unsupported value %q", v)
		}
		if err != nil {
			return nil, false, err
		}
		out = append(out, t)
	}
	return out, allDay, nil
}

func (p *calParser) zone(tzid string) *time.Location {
	if loc, ok := p.zones[tzid]; ok {
		return loc
	}
	loc, err := time.LoadLocation(tzid)
	if err != nil {
		// Some clients prefix the IANA name, e.g.
		// "/freeassociation.sourceforge.net/Tzfile/Europe/Berlin".
		parts := strings.Split(strings.Trim(tzid, "/"), "/")
		if len(parts) >= 2 {
			loc, err = time.LoadLocation(strings.Join(parts[len(parts)-2:], "/"))
		}
	}
	if err != nil {
		p.warn("tz:"+tzid, "unknown TZID %q, using local time", tzid)
		loc = time.Local
	}
	p.zones[tzid] = loc
	return loc
}

func (p *calParser) warn(key, format string, args ...any) {
	if p.warned[key] {
		return
	}
	p.warned[key] = true
	log.Printf("calendar %s: "+format, append([]any{p.file}, args...)...)
}

// calendarDaysBetween counts calendar days from a to b (both local midnights),
// independent of DST shifts.
func calendarDaysBetween(a, b time.Time) int {
	ay, am, ad := a.Date()
	by, bm, bd := b.Date()
	ua := time.Date(ay, am, ad, 0, 0, 0, 0, time.UTC)
	ub := time.Date(by, bm, bd, 0, 0, 0, 0, time.UTC)
	return int(ub.Sub(ua).Hours() / 24)
}

// === Formatting ===

var germanWeekdays = [...]string{"So", "Mo", "Di", "Mi", "Do", "Fr", "Sa"}

// calDayLabel formats a date as "Sa 27.09.".
func calDayLabel(t time.Time) string {
	return fmt.Sprintf("%s %02d.%02d.", germanWeekdays[t.Weekday()], t.Day(), int(t.Month()))
}

// describeRRule turns common rules into short text ("weekly (Mo)",
// "every 2 weeks (Sa)", "monthly (day 1)"). Anything with parts it does not
// know falls back to the raw rule.
func describeRRule(opt *rrule.ROption) string {
	if len(opt.Bysetpos) > 0 || len(opt.Byyearday) > 0 || len(opt.Byweekno) > 0 ||
		len(opt.Byhour) > 0 || len(opt.Byminute) > 0 || len(opt.Bysecond) > 0 {
		return opt.RRuleString()
	}
	units := map[rrule.Frequency][2]string{
		rrule.DAILY:   {"daily", "days"},
		rrule.WEEKLY:  {"weekly", "weeks"},
		rrule.MONTHLY: {"monthly", "months"},
		rrule.YEARLY:  {"yearly", "years"},
	}
	u, ok := units[opt.Freq]
	if !ok {
		return opt.RRuleString()
	}
	text := u[0]
	if opt.Interval > 1 {
		text = fmt.Sprintf("every %d %s", opt.Interval, u[1])
	}
	var details []string
	for _, wd := range opt.Byweekday {
		name := germanWeekdays[(wd.Day()+1)%7]
		if n := wd.N(); n != 0 {
			name = strconv.Itoa(n) + ". " + name
			if n == -1 {
				name = "last " + germanWeekdays[(wd.Day()+1)%7]
			}
		}
		details = append(details, name)
	}
	for _, d := range opt.Bymonthday {
		details = append(details, fmt.Sprintf("day %d", d))
	}
	for _, m := range opt.Bymonth {
		if m >= 1 && m <= 12 {
			details = append(details, germanMonths[time.Month(m)])
		}
	}
	if len(details) > 0 {
		text += " (" + strings.Join(details, ", ") + ")"
	}
	return text
}

// === Diary lines ===

// calDayLines is the calendar content shown under a diary day.
type calDayLines struct {
	Events string // "Feiertag, 10:00 Brunch, 19:30 Kino"
	Weight string // "95,6 kg"
}

var weightTitle = regexp.MustCompile(`(?i)^\s*(\d+(?:[.,]\d+)?)\s*kg\s*$`)

// calendarDayLines builds the diary lines for every day in [from, to), keyed
// by "YYYY-MM-DD". Events come from the diary role, weights from the weight
// role. Days without either are absent.
func calendarDayLines(from, to time.Time) map[string]calDayLines {
	out := map[string]calDayLines{}
	if calendarsDir == "" {
		return out
	}
	s := readCalendarSettings(calendarsDir)

	type dayEvents struct{ allDay, timed []calEvent }
	byDay := map[string]*dayEvents{}
	for _, ev := range calendarOccurrences(calendarsDir, s.Diary, from, to) {
		if ev.AllDay {
			// Multi-day all-day events show on every day they cover.
			for d := ev.Start; d.Before(ev.End); d = d.AddDate(0, 0, 1) {
				key := d.Format("2006-01-02")
				if byDay[key] == nil {
					byDay[key] = &dayEvents{}
				}
				byDay[key].allDay = append(byDay[key].allDay, ev)
			}
			continue
		}
		key := ev.Start.Format("2006-01-02")
		if byDay[key] == nil {
			byDay[key] = &dayEvents{}
		}
		byDay[key].timed = append(byDay[key].timed, ev)
	}
	for key, d := range byDay {
		sort.SliceStable(d.allDay, func(i, j int) bool { return d.allDay[i].Summary < d.allDay[j].Summary })
		var parts []string
		for _, ev := range d.allDay {
			if ev.Summary != "" {
				parts = append(parts, ev.Summary)
			}
		}
		// Timed events are already sorted by start, then summary.
		for _, ev := range d.timed {
			parts = append(parts, strings.TrimSpace(ev.Start.Format("15:04")+" "+ev.Summary))
		}
		if len(parts) > 0 {
			out[key] = calDayLines{Events: strings.Join(parts, ", ")}
		}
	}

	stamps := map[string]time.Time{}
	for _, ev := range calendarOccurrences(calendarsDir, s.Weight, from, to) {
		m := weightTitle.FindStringSubmatch(ev.Summary)
		if m == nil {
			continue
		}
		key := ev.Start.Format("2006-01-02")
		// Several entries on one day: the last edited one wins.
		if prev, ok := stamps[key]; ok && ev.Stamp.Before(prev) {
			continue
		}
		stamps[key] = ev.Stamp
		lines := out[key]
		lines.Weight = strings.ReplaceAll(m[1], ".", ",") + " kg"
		out[key] = lines
	}
	return out
}